1. Field of the Invention
The present invention relates to specimen inspection equipment for analyzing a semiconductor device or the like and how to make electron beam absorbed current images using the same. For example, the invention relates to the technique of identifying a location of electrical failure in a wiring pattern on a semiconductor device or the like.
2. Description of the Related Art
In a semiconductor device on whose semiconductor surface a circuit is formed, it is becoming more difficult to identify a failure location as the device is becoming finer, so that it takes long time to perform the failure analysis. For the analysis, analysis equipment such as OBIRCH (Optical Beam Induced Resistance Change) equipment, an EB tester, or the like has been used at present.
As a failure analysis on a wiring pattern in the failure analysis on the semiconductor device, in recent years, attention is being paid to the technique of irradiating the surface of the semiconductor device with an electron beam, analyzing current absorbed by the wiring pattern or a secondary signal emitted from the semiconductor device, and forming an image from the current/signal.
Japanese Patent Application Laid-Open No. 2002-368049 discloses the technique of identifying a failure location in a semiconductor device by bringing probes into contact with both ends or one end of a pattern, scanning the pattern on the semiconductor device with an electron beam, measuring current flowing in the probes, and forming an image.
Japanese Patent Application Laid-Open No. 2004-296771 discloses the technique of amplifying signals from a plurality of probes, obtaining the difference between the signals, performing a scanning with the differential amplification signals, and displaying an image, and the technique of modulating an electron beam, performing a scanning with the modulated electron beam, and displaying an image.
As described in the conventional techniques, at the time of measuring current outputted from a probe, when one probe is connected to a current amplifier, another probe is grounded, and signals from the probes are measured by the current amplifier, the situation is as follows.
When probes are in contact with both ends of a wiring pattern and a semiconductor device is irradiated/scanned with an electron beam in that state, some of current supplied to the wiring pattern (absorbed current) flows from the point where the electron beam strikes to the ground, and the other flows toward the current amplifier. In this case, the original resistance of the wiring pattern is divided between portions each from the point where the electron beam strikes to the contact point of different one of said probes. The absorbed current supplied from the electron beam to the wiring pattern is bifurcated according to the divided resistance values, and the resultant currents each are passed to either the ground or the current amplifier. With the measuring method, when a failure exists in the wiring pattern, a difference due to the abnormal resistance value can be observed, so that a location of the failure can be identified. However, when the resistance of the pattern is smaller than input impedance of the current amplifier, the absorbed current flows to the ground more than to the current amplifier. When the difference between the resistance of the pattern and input impedance of the current amplifier is large, the difference between absorbed currents each flowing in either the ground or the current amplifier increases, and flow of the absorbed current to the current amplifier is suppressed. Consequently, a wiring pattern having a small resistance value cannot be measured, and a failure location cannot be identified.
Similarly, in the case of the measurement using two probes, inputs of the differential amplifier are connected to the outputs of each amplifier in the conventional configuration. Currents outputted from the probes are amplified by the different amplifiers and, after that, the amplified currents are supplied to the differential amplifier. In this case, the signals which are amplified by the different gain according to the individual difference among the amplifiers connected to the input of the differential amplifier are input to the differential amplifier. As a result, there is a case such that the differential signal between the signals obtained by amplifying the currents input from the probes at their respective different gains is amplified by the differential amplifier, and that a value different from the actual current is measured.
In the case where one of input signals to the differential amplifier is amplified extremely larger than the other signal, the output goes off the scale to the positive or negative side. To avoid such a state of things, the amplifiers have to be adjusted to have the same gain and the same offset. Consequently, measurement itself is also complicated.
Further, since the differential amplifier is used, when the probes come into contact with both ends of a wiring pattern, a loop is formed between input terminals of the differential amplifier through a connection cable from the probes to the input terminals. As the loop is influenced by the magnetic field, in the case where a magnetic shield is not provided, currents determined by both an induced electromotive force generated by the magnetic field and the impedance of the wiring pattern may be directly superimposed as noise upon the signals of the input current from the probes.